1. Field of the Invention
The present invention relates to a power switching circuit for selecting one of a plurality of supply voltages, and providing the selected supply voltage to internal portions of an IC circuit, and more particularly to a power switching circuit for a CMOS circuit operable at a low supply voltage.
2. Description of Related Art
FIG. 2 is a circuit diagram showing a conventional power switching circuit. In this figure, the reference numerals 101-103 each designate an input terminal of a supply voltage VCC (e.g., a normal supply voltage of 5 V), 104 and 105 each designate an input terminal of a supply voltage VSH different from the supply voltage VCC, 106 designates an output terminal of the selected voltage, 107 designates an input terminal of a control signal having a logical value of "1" or "0", 108 designates a power switching portion and 109 designate a level shifter.
The power switching portion 108 comprises a P-channel transistor 121 connected across the input terminal 101 and the output terminal 106, and a P-channel transistor 122 connected across the input terminal 104 and the output terminal 106.
The level shifter 109 comprises a series connection of a diode 123, a P-channel transistor 124 and an N-channel transistor 125 connected across the input terminal 102 and a ground, another series connection of a diode 126, a P-channel transistor 127 and an N-channel transistor 128 connected across the input terminal 105 and a ground, and an inverter 129 connected between the input terminal 107 and the gate of the N-channel transistor 128. The cathodes of the diodes 123 and 126 are connected to the sources of the P-channel transistors 124 and 127, respectively. The gate of the P-channel transistor 124 is connected to the source of the N-channel transistor 128, and the gate of the P-channel transistor 127 is connected to the source of the N-channel transistor 125.
The reference numerals 130 and 131 designate backgates of the P-channel transistors 121 and 122, respectively, which are set in the floating state. The reference numeral 132 designates a node (connecting point) interconnecting the sources of the P-channel transistors 124 and 127, 133 designates a node interconnecting the drain of the P-channel transistor 124 and the source of the N-channel transistor 125, and 134 designates a node interconnecting the drain of the P-channel transistor 127 and the N-channel transistor 128.
Next, the operation will be described.
First a case will be described where VCC&gt;VSH, and the control signal is set at logic "1" to select the supply voltage VCC. In this case, the N-channel transistor 125 is brought into conduction, while the N-channel transistor 128 is brought out of conduction by the output of the inverter 129. The potential of the node 133 thus set at the ground level is supplied to the gates of the P-channel transistors 121 and 127, and causes them to conduct.
Thus, the potential of the node 134 is set at the potential of the node 132 through the conducting P-channel transistor 127. Since the node 132 is connected through the diodes 123 and 126 to the input terminals 102 and 105, to which the supply voltages VCC and VSH are applied, respectively, no current flows toward the input terminals 102 and 105, and the higher voltage VCC cuts off the diode 126, thereby supplying the node 134 with the higher voltage VCC.
More precisely, the potential of the node 134 becomes (VCC-.DELTA.Vd), a value obtained by subtracting from the supply voltage VCC the forward drop voltage .DELTA.Vd of the diode 123, which is about 0.5 V, because the diodes 123 and 126 drop their cathode voltages due to the effect of the forward voltage. Thus, the P-channel transistor 122 and the P-channel transistor 124 are brought out of conduction, and the P-channel transistor 121 is brought into conduction, resulting in the output of the supply voltage VCC which is applied to the input terminal 101 from the output terminal 106.
Although the foregoing illustrates the case where VCC&gt;VSH, a similar description holds true when VCC&lt;VSH.
FIG. 3 is a cross-sectional view of the P-channel transistors 121 and 122. When the P-channel transistor 121 is conducting, the potential VCC is conducted from an N-well 141 to an N-well 142 through a wire 136 because the backgates 130 and 131 are placed in the floating state. Accordingly, the potential of the N-well 142 becomes VCC if VCC&gt;VSH, whereas it is maintained at VSH if VCC&lt;VSH. On the other hand, when the P-channel transistor 122 is conducting, the potential VSH is conducted from the N-well 142 to the N-well 141 through the wire 136. Thus, although the potential of the N-well 141 is maintained at VCC, if VCC&gt;VSH, it becomes VSH if VCC&lt;VSH.
When the potential of the source (P-region) becomes higher than that of the N-well in an ordinary P-channel transistor in which the backgate is connected to a power supply, a current will flow from the P-region to the N-well due to the forward voltage across the PN junction. In view of this, it is preferable in the P-channel transistor that the potential of the backgates determining the potential of the N-well be set equal to the potential of the source. The power switching portion 108 as shown in FIG. 1, however, is supplied with two voltages of different potentials, and hence the potential of the output terminal 106 varies depending on whether the VCC or VSH is selected.
When selecting the supply voltage VCC, the P-region at the output terminal 106 side becomes the source of the P-channel transistor 122 if VCC&gt;VSH, whereas the P-region at the input terminal 104 side becomes the source of the P-channel transistor 122 if VCC&lt;VSH, so that setting the backgates 130 and 131 in a floating state makes it possible to handle either case.
Since the conventional power switching circuit is configured as described above, the potential of the level shifter 109, which supplies the gate potential to the unselected P-channel transistor of the two P-channel transistors 121 and 122, becomes lower than the potential of the higher one of the supply voltages VCC and VSH by an amount equal to the forward drop voltage of the diodes 123 and 126.
To cut off the P-channel transistors 121 and 122 completely, their threshold voltage must be set higher than the forward drop voltage of the diodes 123 and 126 because they are thoroughly cut off only when their source-to-gate voltage is lower than the threshold voltage.
If the threshold voltage of the P-channel transistors 121 and 122 constituting the power switching circuit is set lower than the forward voltage drop of the diodes 123 and 126, the unselected P-channel transistor cannot be cut off completely. As a result, a current flows from the higher to the lower one of the two supply voltages VCC and VSH. This presents a problem in that the supply voltage output from the power switching circuit becomes lower than the higher supply voltage when it is selected, or becomes higher than the lower supply voltage when it is selected.